Method of polymer flooding

ABSTRACT

Polymer flooding to increase production of formation fluids from a subterranean oil-bearing reservoir of low permeability, utilizing a polymer of a specified average molecular weight with at least 5 percent by weight having an average molecular weight at least 1.2 times greater than this specified average molecular weight is improved by mechanically or physically degrading the polymer solution. This substantially decreases the tendency of the polymer solution to &#39;&#39;&#39;&#39;plug&#39;&#39;&#39;&#39; the formation, while only slightly decreasing its viscosity.

Un1te il ,.States Patent 11 1 1 1 3,726,342 Rl'mdy et a1. 51 Apr. 10,1973 [54] METHOD OF POLYMER FLOODING 2,827,964 3/1958 Sandiford et al...166/27S x 3 305 016 2/1967 Lindblom et a1. 166/246 [75] Inventors:John S. Rhudy; Gerald W. Haws,

both of Denver, Colo. 3,406,754 10/1968 Gogarty ..l66/273 [73] Assignee:Marathon Oil Company, Findlay, Primary Examiner-Stephen .I. Novosad OhioAttorney-Joseph C. Herring, Richard C. Willson, Jr. 221 Filed: Mar. 24,1971 and Jack Hume [21] Appl.No.: 127,483 [57] ABSTRACT I Polymerflooding to increase production of formation [52] US. Cl "166/275 fluidsfrom a subterranean oil-bearing reservoir of low [51 1111. C1. ..E2lb43/22 p r ability, utilizing a polymer of a specified average [58] FieldM Search ..l66/275, 274, 273, m l lar weight with at least 5 percent byweight 166/268, 305 R; 252/8-55 D having an average molecular weight atleast 1.2 times greater than this specified average molecular weight isReferences Cited improved by mechanically or physically degrading theUNITED STATES PATENTS polymer solution. This substantially decreases thetendency of the polymer solution to plug the forma- 3,039,529 6/1962McKennon 166/275 1 tion, while only slightly decreasing its viscosity.3,371,714 3/1968 Katzcr ....166/275 X 3,180,410 4/1965 Turbak 166/275 12Claims, 2 Drawing Figures PoLYusR vlscoslrv Am scnezu FACTOR oeenmmm m A"vmns" 11014005111251: AT wuous s asm "vmns". 11011005111251? (RPM)PATEHTEDAPRIOIQB 3,726,342

SHEET 1 OF 2 Fig.1

POLYMER VISCOSITY AND SCREEN FACTOR DEGRADATION IN A "vIRTIs"HOMOGENIZER AT \ARIous S'PEEDS I00 I I g 90 25 g I- 80 LI.

2 NORMALIZED VISCOSITY w 70 (INITIAL VISCOSITY/FINAL a: VISCOSITY) U (DQ w 60 '1' .J z

I- I m 50 o z 5 40 I; NORMALIZED SCREEN 8 30 FACTOR (FINAL SCREEN oFPCTOR/INITIAL SCREEN g FACTOR) O m 20 L. J 1 5 IO 0 l l l 0 5,000Io,0o0 I5,000

"VIRTIs" HOMOGENIZER (RPM) INVE/VTG? JOHN S. RHUDY GERALD W. HAWS /ZZZMAT T ORA/E Y SHEET 2 0F 2 NORMALIZED VISCOSITY pgagg x uzeo SCREEN ig. 2POLYMER VISCOSITY AND SCREEN FACTOR DEGRADATION AT \ARIOUS FLOW RATESTHROUGH A 0.06 D. CAPILLARY TUBE IOO PATENTLU APR 1 0191s INVENTOI? JOHNS. RHUDY GERALD W. HAWS ATTORNEY O.| 0.2 0.06" 0. CAPILLARY TUBEFLOWRATE (cc /SEC) METHOD OF POLYMER FLOODING BACKGROUND OF THEINVENTION l. Field of the Invention This invention relates to thesecondary and tertiary recovery of oil from a subterranean oil-bearingreservoir having at least one injection means in fluid communicationwith at least one production means. More particularly, it relates to animproved method of polymer flooding.

2. Description of the Prior Art Secondary and tertiary recovery of oilby injection of an aqueous flooding medium into a subterraneanoilbearing formation through an injection well which is in fluidcommunication with a producing well is a wellknown process. Theincorporation of a viscosity-increasing agent within the front portionof a waterflood provides a favorable viscosity condition, resulting inless fingering and improved oil recovery. The front portion of thewaterflood preferably has a mobility which is equal to or slightly lessthan the mobility of the crude oil or formation fluids.

US. Pat. No. 2,827,964 to Sandiford teaches a secondary recovery processin which a viscous aqueous solution of a water-soluble partiallyhydrolyzed acrylamide polymer is injected into an injection well andforced through the formation toward a production well.

US. Pat. No. 3,039,529 to McKennon teaches that about 100 to about 5,000ppm of a high molecular weight partially hydrolyzed polyacrylamide isuseful to improve waterflooding for recovering crude oil from asubterranean oil-bearing formation.

US. Pat. No. 2,771,138 to Beeson teaches a waterflooding method whereinmobility reducing agents, such as naturally occurring gums and polymers;aqueous solutions containing synthetic polymers, e.g., copolymers ofmethyl vinyl ether and maleic anhydride, condensation products of fattyacids and hydroxy amines, sodium polyacrylide, polyacrylic acid, sodiumpolymethyl acrylate, etc.; sucrose and crude sugar, etc., are useful torecover crude oil from subterranean oil-bearing formations.

US. Pat. No. 3,282,337 to Pye teaches a waterflooding process whichemploys dilute and comparatively non-viscous aqueous solutions ofcertain water soluble polymers.

Preferable mobility reducing agents are high molecular weight polymers.However, the utilization of these high molecular weight polymers in asubterranean oilbearing formation of low permeability often creates aserious plugging problem. As a result, much of the reservoir oil isby-passed by the waterflood and/or a substantial decrease in injectivityindex is realized.

SUMMARY OF THE INVENTION Applicants have discovered that improved oilrecovery can be realized in the flooding of subterranean oil-bearingreservoirs of low permeability by mechanically or physically degradingthe polymer solution prior to flooding. This mechanical or physicaldegradation causes a substantial decrease in the screen factor of thepolymer solution, while only slightly reducing its viscosity. Thetendency of the polymer solution to plugthe formation is thereforesubstantially decreased.

2 i DESCRIPTION OF THE DRAWINGS FIG. 1 presents a comparison of therespective decreases of normalized viscosity and normalized screenfactor when each is plotted against increasing shear force. This iseffected by incrementally increasing the speed ofa 1.5 inch bladediameter Virtis 45 homogenizer. The solution consists of distilled watercontaining 400 ppm of Dow 700 Series Pusher polymer (a partiallyhydrolized polyacrylamide marketed by Dow Chemical Co., Midland,Michigan). The viscosities are measured on a Brookfield LVT viscometer.The terms normalized screen factor and normalized viscosity arediscussed infra, pp. 5 and 6, respectively.

FIG. 2 presents a comparison similar to FIG. 1 in all respects exceptthe means of degradation. For this illustration the polymer solution wasdegraded by flowing it through a 0.06 inch diameter capillary tube 4.96cm in length.

DESCRIPTION OF THE INVENTION The polymers which can be improved by themechanical or physical degradation of this invention are those whichhave an average molecular weight of about 500,000 to about 50,000,000 ormore, with at least about 5 percent by weight of the polymer having anaverage molecular weight at least about 1.2 times greater than theaverage molecular weight of the overall polymer. Polymers within thismolecular weight range may possess a relatively high degree ofviscoelasticity.

Specific examples of polymers which can be improved by this inventioninclude high molecular weight polyelectrolytes, specifically highmolecular weight polyacrylamides and partially hydrolyzed productsthereof, sulfonated high molecular weight polymers, chemically modifiedpolymers such as CMC (carboxy methyl cellulose), polysaccharides, andlike polymers. A preferred mobility reducing agent is the Pusher polymersold by Dow Chemical Company, Midland, Michigan.

The term degradation as used herein is intended to mean the actualcracking or shearing of those molecules which have molecular weightsgreater than about 1.2 times the average molecular weight of thepolymer.

Mechanical or physical means of degradation are apparatus or processes,respectively, which impart high shear forces to the polymer. These highshear forces actually crack or shear those molecules which comprise theupper, about 5 percent by weight, molecular weight range of the polymer.Examples of apparatus useful for shearing include high pressure pumps,mechanical mixers, homogenizers, etc. An example of a physical means ofshearing is a piping arrangement, with possibly undersized pipes,characterized by numerous bends and/or valves. Physical parameters suchas temperature, pressure, flow rate, etc., are, of course, intrinsicallyrelated to these degradation means. The degradation can take placewithin the wellbore, i.e. before the polymer enters the reservoir.However, the polymer is preferably degraded prior to its introductioninto the wellbore.

I The term screen factor as used herein is defined as the ratio ofpolymer flow time to water flow time through an apparatus which providesgravity flow across stainless steel screen(s) of a predetermined mesh.Screen factor relates to the degree of plugging of reservoir formationsby polymer flow because it is sensitive to changes in molecular weightand structure. (See R. R. Jennings et al., Factors Influencing MobilityControl by Polymer Solution, SPE 2867, p. 162, May 14-15, 1970, for amore detailed description of this term.) The term normalized screenfactor" as used herein is defined as the quotient of the screen factorof the polymer solution after degradation divided by the screen factorof the polymer solution before degradation.

The mechanism of the plugging is possibly a combination of actualretention of a very small amount of the polymer within the porous mediumand the use of a relatively high viscoelastic polymer. Irrespective ofthe mechanism, however, it is asserted that most of the plugging occursas a result of that component of the polymer which represents thegreatest molecular weight.

An important aspect of this invention is that the polymer, after beingsubjected to the mechanical or physical degradation, exhibits asubstantially lower screen factor with only a slight decrease inviscosity. Therefore, the polymer maintains its desirable floodingproperties, while dismissing its undesirable plugging characteristics.

EX AMPLEI Samples A, B, and C are prepared by adding 1,000 ppm of Dow530 Pusher polymer (partially hydrolized polyacrylamide) to fresh water(about 500 ppm total dissolved solids). The screen factor of sample A isdetermined via a screen viscometer, described by Jennings et al.(supra). About pore volumes of Sample A is then injected into areservoir core plug about 1 inch in diameter by about 3 inches long. Thecore is effectively divided into front and rear sections by locat-. inga pressure tap about 1 inch from the injection face. The absolutepermeability of the core sample to water is measured before and afterthe injection of the polymer solution. A plugging factor, termedpermeability reduction factor", is then calculated by dividing thepermeability of the core before polymer injection by the permeability ofthe core after polymer injection.

Samples B and C are handled in the same manner as Sample A, however,prior to core injection, they are degraded for about 2 minutesin aVirtis 45" homogenizer (blade diameter 1.5 inches; this is essentially ablade type mixer) run-at about 2,500 rpm. viscosities are measuredbefore and after the degradation on a Brookfield LVT viscometer equippedwith a U/L adaptor, running at 6 rpm (which corresponds to a shear rateof 8 sec). These viscosities are reported as normalized by dividing theafter" viscosity by the before viscosity.

The results of these samples are reported in Table I.

The results indicate a reduction in screen factor of about 0.31 to 0.48,with a corresponding reduction in viscosity of only about 0.08. Withrespect to permeability reduction the differences between degradedSamples B and C and undegraded Sample A clearly show a major improvementin the plugging of polymer flooding on core samples.

EXAMPLE II This example compares the normalized screen factor reductionto the corresponding reduction in normalized viscosity. A solution ofdistilled water containing 400 ppm Dow 700 Series Pusher polymer (apartially hydrolyzed polyacrylamide) is degraded for 2 minutes in aVirtis 45 homogenizer at various incremental speeds. The screen factorsand viscosities are measured in the same manner as Example I for eachspeed. The results, graphically illustrated in FIG. 1, indicate thescreen factor is reduced considerably more, e.g., at

least 35 percent, than the corresponding viscosities.

EXAMPLE Ill The same process followed in Example II is followed in thisexample. However, the solution is degraded by flow through a 0.06 inchdiameter capillary tube 4.96 inches long at various flow rates. Theresults, graphically illustrated in FIG. 2, again indicate an averagereduction in screen factor which is considerably more, e.g., at least 40percent, than the corresponding reductions in viscosity.

It should be understood that the invention is capable of a variety ofmodifications which will be made apparent to those skilled in the art bya reading of the specification and which are to be included within thespirit of the claims appended thereto.

What is claimed is:

1. In a process for recovering crude oil from a subterranean oil-bearingreservoir having at least one injection means in fluid communicationwith at least one production means and wherein an aqueous solution of ahigh molecular weight polymer is injected into the formation tofacilitate the flow of crude oil toward a production means and whereinthe polymer has a predetermined average molecular weight with at least 5percent by weight of the polymer having an average molecular weight atleast about 1.2 times greater than the average molecular weight of thetotal polymer within the aqueous solution and wherein the atleast 5percent portion contributes significantly to reducing the permeabilityof the subterranean oil-bearing formation, the improvement comprisingsubjecting the polymer solution to sufficient shear force to shearsubstantially all of the at least 5 percent portion of the polymer,thereby reducing the average molecular weight of the at least 5 percentportion of the polymer and thereafter injecting the polymer into thereservoir, the overall effect being to inhibit the tendency of thepolymer to substantially reduce the permeability of the reservoir.

2. The process of claim 1 wherein the high molecular weight polymer hasa molecular weight of from about 500,000 to about 50,000,000.

- 3. The process of claim 1 wherein the subterranean oil-bearingreservoir is characterized as having an average permeability of about 5to about 200 millidarcies.

4. The process of claim 1 wherein the high molecular weight polymer ischaracterized as being viscoelastic.

5. The process of claim 1 wherein the polymer is a partially hydrolyzed,high molecular weight polyacrylamide. i

6. The process of claim 1 wherein the polymer is sheared prior to itsintroduction into the injection means.

7. In a process for recovering crude oil from a subterranean oil-bearingreservoir having at least one injection means in fluid communicationwith at least one production means and wherein an aqueous solution of ahigh molecular weight polymer is injected into the formation tofacilitate the flow of crude oil toward a production means and whereinthe polymer has a predetermined average molecular weight with at least 5percent by weight of the polymer having an average molecular weight atleast about l.2 times greater than the average molecular weight of thetotal polymer within the aqueous solution and wherein at least the 5percent portion contributes significantly to reducing the permeabilityof the subterranean oil-bearing reservoir, the improvement comprisingsubjecting the polymer solution to sufficient shear force tosubstantially reduce the screen factor of the polymer while onlyslightly reducing the average molecular weight of the polymer, theoverall effect being to reduce the tendency of the polymer to reduce thepermeability of the reservoir.

8. The process of claim 7 wherein the high molecular weight polymer hasa molecular weight from about 500,000 to about 50,000,000.

9. The process of claim 7 wherein the subterranean oil-bearing reservoiris characterized as having an average permeability of about 5 to about200 millidarcies.

10. The process of claim 7 wherein the high molecular weight polymer ischaracterized as being viscoelastic.

11. The process of claim 7 wherein the polymer is a partiallyhydrolyzed, high molecular weight polyacrylamide.

12. The process of claim 7 wherein the polymer is sheared prior to itsintroduction into the injection means.

2. The process of claim 1 wherein the high molecular weight polymer has a molecular weight of from about 500,000 to about 50, 000,000.
 3. The process of claim 1 wherein the subterranean oil-bearing reservoir is characterized as having an average permeability of about 5 to about 200 millidarcies.
 4. The process of claim 1 wherein the high molecular weight polymer is characterized as being viscoelastic.
 5. The process of claim 1 wherein the polymer is a partially hydrolyzed, high molecular weight polyacrylamide.
 6. The process of claim 1 wherein the polymer is sheared prior to its introduction into the injection means.
 7. In a process for recovering crude oil from a subterranean oil-bearing reservoir having at least one injection means in fluid communication with at least one production means and wherein an aqueous solution of a high molecular weight polymer is injected into the formation to facilitate the flow of crude oil toward a production means and wherein the polymer has a predetermined average molecular weight with at least 5 percent by weight of the polymer having an average molecular weight at least about 1.2 times greater than the average molecular weight of the total polymer within the aqueous solution and wherein at least the 5 percent portion contributes significantly to reducing the permeability of the subterranean oil-bearing reservoir, the improvement comprising subjecting the polymer solution to sufficient shear force to substantially reduce the screen factor of the polymer while only slightly reducing the average molecular weight of the polymer, the overall effect being to reduce the tendency of the polymer to reduce the permeability of the reservoir.
 8. The process of claim 7 wherein the high molecular weight polymer has a molecular weight from about 500,000 to about 50, 000,000.
 9. The process of claim 7 wherein the subterranean oil-bearing reservoir is characterized as having an average permeability of about 5 to about 200 millidarcies.
 10. The process of claim 7 wherein the high molecular weight polymer is characterized as being viscoelastic.
 11. The process of claim 7 wherein the polymer is a partially hydrolyzed, high molecular weight polyacrylamide.
 12. The process of claim 7 wherein the polymer is sheared prior to its introduction into the injection means. 